The invention relates to communication facilities, wherein the transmission paths can be employed for different types of message switching, for example, voice communication by telephone and data communication. To satisfy the varying conditions in the switching facilities of such systems in the most economical way, there are provided various connector sets over which the individual calls are conducted. The connector sets can be selected in accordance with the type of message switching desired through a code selected prior to the call number identifying the desired distant station. As a rule, there are provided in the subscriber stations of such systems changeover facilities connecting the subscriber lines to one of the terminal units for the individual types of message switching.
Various modes of operation have been proposed for the control of the changeover means, for example, for voice and data communication.
In one of these modes of operation a distinction is made between remote stations and central stations. The central stations are equipped with a mode selector switch, so that the changeover means can be switched manually to the data terminal unit by operating a data button after the call has been placed, as well as directly by the call signal in the case of automatic operation. Upon interrogation of data of a remote station by the central station, automatic operation is not possible. It might, however, be possible to equip the remote stations also with a mode selector switch and with an automatic call discriminating means, as in the central stations. However, changeover by means of the call signal is a disadvantage in that upon setting the mode selector switch to automatic operation no incoming telephone traffic can be accepted, since the forward signal causes automatic switching to the data communication mode.
Another mode of operation for prior art telephone installations provides changeover facilities without a mode selector switch. The changeover to the data communication mode takes place by selectively operating a data button in one of the subscriber stations participating in a call. An appropriately equipped connector set detects this request and sends to both subscriber stations a changeover signal, whereby a switch back or a release of the connection can only be effected by the initiating subscriber station. Although this solution is advantageous in many respects, it differs substantially from the more common mode of operation referred to at the outset, and fully automatic operation is not possible.
It is an object of the invention to provide apparatus capable of a mode of operation which assigns priority to one type of message switching, viz. voice transmission and permits the selection between automatic and manual operation.
A further object is to provide such apparatus which is compatible with the other known mode of operation, independently of the selected type of transmission for the data communication, and wherein the number of operation possibilities is increased.